THE SATURN NEBULA
"Of wonderfully intricate structure" (Curtis), the planetary nebula NGC 7009, the "first
planetary," displays a pair of jug-handle-like "ansae" that give it
the name "The Saturn Nebula." On the left is a drawing made from
a series of photographs, the gaseous nebula perhaps appearing as
its discoverer William Herschel may have seen it.
In Herschel's words about his discovery object (from an article in
the Philosophical Transactions of the Royal Society in
1785): "I shall conclude this paper with an account of a few
heavenly bodies, that from their singular appearance leave me
almost in doubt of where to class them. The first precedes Nu Aquarii 5'.4 minutes in time [5.4 minutes,
1.25 degrees], and is I' (minute of arc) more north ... The
planetary appearance of the first two is so remarkable, that we can
hardly suppose them to be nebulae; their light is so uniform, as
well as vivid, the diameters so small and well defined, as to make
it almost improbable that they should belong to that species of
bodies." Thus was born the oft-confusing term, by which Herschel
merely meant "disk-like." After all, who had a better right than
the discoverer of the first planet since ancient times? (While Lyra's Ring Nebula
was already known, it was not placed into the category of planetary
nebulae until later.) Herschel then went on to discover (in NGC 6543) that planetary nebulae have central
On the right is a Hubble Space Telescope image made nearly a
century later, showing the vast improvement in astronomical imagery
as well as the immense complexity of the nebula. The hot (90,000
Kelvin), blue, 13th magnitude star at the center is the old
nuclear-burning core of what was once an extended giant star, while
the surrounding nebula is the inner part of the star's lost
envelope that has been structured by the remaining star's hot wind.
The distance is not well known. If at 2000 light years, the nebula
is 3/4 of a light year long. The central star seems faint to us
only because of its distance and because the vast majority of its
radiation comes out as energetic ultraviolet light. In reality,
the star thousands of times more luminous than the Sun. Still
heating at constant luminosity, the star will eventually cool and
dim to become one of the many white dwarfs that dot the cosmos.
The ansae, called "FLIERS," are high-speed jets shot out of the
central star, probably along the star's rotation axis. While NGC
7009's are the most prominent, they are seen in a number of
nebulae, notably NGC 3242, NGC 6543, and NGC 2371-
2. Their origins and significance are not understood.
Left: Image and quote by H. D. Curtis from Publications of the Lick
Observatory, Volume 13, Part III, 1918. Right: B. Balick, J.
Alexander (U. Wash.), A. Hajian (USNO), Y. Terzian (Cornell U.), M.
Perinotto (U. Florence), P. Patriarchy (Arcetry Obs.), and NASA.